All structures and machinery components undergoing fatigue loading are prone to crack formation and its subsequent growth that increases with time. When a crack is formed, the strength of the structure or the component is decreased and can no longer function in the intended manner for which it was designed for. Moreover, the residual strength of the structure decreases progressively with increasing crack size. Eventually, after a certain time the residual strength becomes so low that the structure fails [1]. It is, therefore, of paramount importance to be able to predict the rate of decline in the component's residual strength and the remaining life of the system.
Fracture mechanics is a branch of science that provide insights into the mechanism of failure and help predict the service life of structures and machinery components [1]. As depicted in FIG. 1, several disciplines are involved in the development of fracture mechanics. At the right end of the scale is the engineering load-stress analysis. Applied mechanics covers the analysis of crack tip stress fields as well as the elastic and plastic deformations of the material in the vicinity of the crack. Material science concerns itself with the fracture processes on the scale of atoms and dislocations in the form of impurities and grains.
In order to make a successful use of fracture mechanics in an engineering application, it is essential to have some knowledge of the total field shown in FIG. 1. Fatigue failure can occur only if—as a result of the presence of micro-cracks, local yielding, micro-cavities, etc.—the applied load produces an increase in the stress in a point (or a zone) of the material, with local values exceeding the elastic limit [2]. It is known that if the stress is static, the local plasticization and the redistribution of the stress onto the surrounding material does not generate any particularly critical condition and the material reaches failure only under decidedly greater loads. On the contrary, in the case of cyclic loading, where the stress is one of fatigue, the material arrives at the condition of local yielding (micro-plasticization) and a micro-crack is generated. Hence, the repeated application of the stress leads to the crack propagation until, eventually, the condition of failure is reached and the specimen breaks.
The thermoelastic effect, which governs the relationship between the temperature variation and stress (or strain) change in the elastic range, has been well documented, and has been utilized to characterize the elastic stress field. Different means—such as thermocouples, thermistors, and thermography techniques—have been employed to monitor the temperature changes during mechanical tests [3-6]. The thermoelastic stress analysis by thermography is now an advanced full-field stress measurement method. In materials undergoing cyclic loading, most of the dissipated energy due to hysteresis effects manifests itself as heat, and the heat is removed from the material by heat transfer.
Heat can be transferred by three processes: conduction, convection, and radiation. Conduction is the transfer of heat along a solid object. Convection transfers heat from the “wetted area” of a solid through the exchange of hot and cold molecules, e.g., air, water, etc. Radiation is the transfer of heat via electromagnetic (usually infrared, IR) radiation. Although these three processes can occur simultaneously, it is not unusual for one mechanism to overshadow the other two. If the fatigue experiment is rapid enough, which is generally true for low-cycle fatigue testing, the temperature rise can be surprisingly high. For fatigue tests at 1,000 Hz, for example, the temperature could increase 200° to 400° K. above the initial temperature, depending on the material tested and specimen geometry [3, 4].
The temperature evolution resulting from the heat generated during the fatigue process is utilized to monitor the fatigue-crack propagation [5-8], to measure the energy required to produce a unit area of a fatigue crack by propagation [8], to determine the endurance limit of some materials [10, 11], and to characterize the evolution of cumulative damage in the fatigue process [3, 4, 12, 13].
In the present invention, a novel approach of nondestructive thermographic technique is used to characterize the fatigue behavior of metals. Specially, laboratory tests were conducted with Aluminum alloy and Stainless Steel undergoing cyclic bending and torsion loads. The same trend is expected to persist in multiaxial loading involving the combination of bending, tension and compression as well as torsion. In the laboratory tests, detailed temperature distributions on the specimen surface, and temperature changes as a function of time (cycles) were obtained. A two-dimensional form of a thermal-mechanical coupling model for a low-cycle bending fatigue was formulated to ensure the validity of the experimental results and to provide insight into the complex fatigue behavior. The results of the experimental and analytical works were used to develop a new method for predicting the fatigue life. The predictions of temperature changes during fatigue were found to be in good agreement with the experimental results.
In materials undergoing cyclic loading, most of the dissipated energy due to hysteresis effect manifests itself as heat and causes an increase in the mean temperature. An abrupt temperature rise in the first few cycles, followed by a steady state in later cycling, is a characteristic of metals that undergo the high-stress level fatigue testing.
In particular, we have determined that slope of the temperature-versus-time curve at the beginning of the test can be effectively utilized as an index for fatigue life prediction. This invention is expected to be applicable for the axial tension/compression loading and torsion of solid specimens of variety of shapes, as well as a thin-walled tube. Therefore, a temperature sensor, either contacting (e.g., thermocouple) or non-contacting (e.g., fiber optic, IR camera), can be used to measure the surface temperature of the specimen under cyclic loading. Test results obtained using the invention used a non-contacting sensor. In this arrangement, the need for measuring the dissipation energy due to plastic deformation from the hysteresis loop is eliminated. Also, this invention can provide an early prediction of the service life of machinery components under cyclic loading. The material properties and thermal boundary conditions are the input parameters and the service life time of the specimen is the output. Furthermore, for a system already in service, this device enables us to determine the remaining life.
Laboratory experimental results conducted at the Center for Rotating Machinery at Louisiana State University have confirmed the validity of this invention for the case of cyclic bending and torsion loads. A thermographic technique that utilizes an IR-camera (i.e., non-contacting method) was used to measure the temperature increase in the specimen due to hysteresis heating during fatigue testing. Similar results can be obtained using fiber optic temperature sensors where temperature can be recorded from a machine remotely.
A miniature electronic chip may be attached to the surface of a specimen under cyclic loading to measure its temperature and process the data to predict the onset of catastrophic failure. This device will be capable of measuring the slope of the temperature curve at the very early stages of the cyclic loading and rapidly estimate the specimen's fatigue life. For a new component, this information would pertain to the fatigue life; for an existing machine in service, it would provide estimate of the remaining life. This instrument provides a very fast and reliable method for the determination of the service life of the machinery components under cyclic loading and torsion. In practical applications, wireless technology provides compact, lightweight, reliable data transfer from the device that can be remotely monitored and processed in real time to predict the number of cycles for fatigue failure. An illustration showing the use of a wireless sensor and a data acquisition unit is shown in FIG. 2.